Apparatus for controlling the pressure force in multi-roll drives for equipment withdrawing an ingot from a continuous casting machine along a curved path

ABSTRACT

A withdrawal stand for continuously cast, curved ingots is disclosed wherein the contact pressure of driven rolls is controlled to normally exceed only slightly the internal pressure of the ingot; the contract pressure is increased if the traction resistance increases.

0 United States Patent 1191 1111 3,776,298

Vogt 1451 Dec. 4, 1973 APPARATUS FOR CONTROLLING THE PRESSURE FORCE IN MULTI-ROLL [56] References Cited DRIVES FOR EQUIPMENT WITHDRAWING UNITED STATES PATENTS AN INGOT FROM A CONTINUOUS 3,263,284 8 1966 Orr et al. 164/282 ux CASTING MACHINE ALONG A CURVED 3,438,425 4/1969 Butkevich et al. 164/282 X PATH 3,543,830 12/1970 Baumann 164/82 3,707,180 12/1972 Vogt 164/82 [751 Invenm" Gem 8 Meerbusch, Gemmy 3,557,865 1 1971 Gallucci.. 164 82 [73] Assignee: Mannesmann Aktiengeseuschafl, 3,638,713 2/1972 Knell 164/282 X Dusseldorf, Germany Primary Examiner-R. Spencer Annear [22] Flled: June 1972 Attorney-Ralf H. Siegemund et a1. [21] Appl. No.2 267,317

. [57] ABSTRACT [30] Foreign Application Priority Data A withdrawal stand for continuously cast, curved in- J l 2 1971 G P 21 33 937 7 gets is disclosed wherein the contact pressure of u y emany driven rolls is controlled to normally exceed only slightly the internal pressure of the ingot; the contract [52] US. Cl. 164/154, 164/282 pressure is increased if the traction resistance In- [51] Int. Cl 822d 11/12 creases 4 [58] Field of Search 164/82, 154, 282,

164/283 S 3 Claims, 1 Drawing Figure APPARATUS FOR CONTROLLING THE PRESSURE FORCE IN MULTI-ROLL DRIVES FOR EQUIPMENT WITHDRAWING AN .INGOT FROM A CONTINUOUS CASTING MACHINE ALONG A CURVED PATH BACKGROUND OF THE INVENTION The present invention relates to an apparatus for control of the pressure applied, for example, hydraulicallyto the rolls in a withdrawal train which has dual rolls and withdraws a continuously cast ingot from the casting mold. More particularly,'the invention relates to equipment of this type wherein the exerted pressure force is larger than the expansion force exerted by the casting upon the respective roll.

Equipment of the type referred to above is generally known, and the knowndevices include particularly adjusted pressure reducing devices, for reducing the hydraulic pressure imparted upon the roll, so that the resulting contact pressure exerted by the roll upon the casting is predetermined primarily on the basis of cal culation and experience which went into the design of the equipment.

The pressing force is selected in these cases so that the casting will be pulled out of the mold at the required rate even if one or several of the many rolls are blocked for one reason or another; for example, because of damage to the bearings or the like. Therefore, the contact pressure exerted by any individual roll upon the casting is higher than required for undisturbed operation of all rolls. On the other hand, this apparent need for providing a larger contact pressure than actually needed, just because of the possible need for compensation and as margin of safety is quite disadvantageous from a different point of view. These rolls as pressing on the ingot at a high rate of force may cause an undesired reduction in the thickness dimension of the casting by roller action. It may even occur that the partially liquidous casting is subjected to fissures or cracks developing on the inside; i.e., in or near the interface between the already solidified skin portion of the liquidous interior. This is even more dangerous in case the cast ingot is relatively wide.

It has been suggested to provide as many driven rolls so that the pressure force of each individual roll is in the range between 100 and 120 percent of the hydrostatic pressure in and of the ingot. It was found that under such circumstances the dangers outlined above are minimal, while on the other hand, there is a sufficient margin of safety available, so that in case of dropout of some of the rolls, the ingot will still be withdrawn. However, it has to be observed that in such a withdrawal equipment, it is by no means necessary that all rolls are being driven. Most rollsin the train are usu ally undriven upPOrt and guide rolls. It appears, therefore, that the pressure force provided by any individual roll can be reduced for reasons of the inherent danger to the ingot only if the number of driven rolls is increased to offset the possibility of failure of one or severa] of the rolls. However, increasing the number of driven rolls in the train constitutes an added expense.

DESCRIPTION OF THE INVENTION It is an object of the present invention to provide ingot and casting withdrawal equipment which permits employment of the usual number of rolls without, however, requiringtexcessively high contact pressure to be exerted by these rolls upon the casting. It is another object of the present invention to teach a mode and manner of improving already existing continuous casting machines, having a particular number of driven rolls, and to provide supplemental equipment that permits reduction of the pressure exerted by these driven rolls upon the casting; these particular, already existing casting machines cannot be equipped with additional .driven rolls.

In accordance with the preferred embodiment of the present invention, it is suggested to provide pressure regulating means for the hydrostatic pressure used to impart pressure force upon the driven rolls for obtaining particular contact pressure on the ingot. The pressure regulating means is to be under control of a variable command input which is determined in accordance with the traction and pulling force needed for withdrawing the continuously cast ingot at a particular rate. In particular, the command value for the hydrostatic pressure regulating means will increase if the drive and pulling force needed is increased, and such increase has been sensed and has occurred; e.g., as a result of dropout of one or more of the withdrawal rolls.

The driven rolls are usually driven by electric motors. Traction and pulling force of the withdrawing equipment is usually controlled for a constant travel rate of the ingot commensurate with a constant rate of casting. The current fed to the motors will increase above the normal value if one or more of the rolls have dropped out. Therefore, the sum of the motor currents is preferably used to generate the command value that controls the pressure regulator or regulators for theseveral hydraulic means that impart pressure upon the rolls.

Some casting machines and withdrawal stands are constructed that several hydraulic piston-cylinder devices are controlled individually or in small groups. In this case, it may be necessary to provide as many pressure regulators as there are hydraulic drives. In order to simplify the construction, it may thus be of advantage to provide such pressure control only to the rolls in the horizontal train portion as the hydrostatic pressure will be the same along that path so that these rolls as they are pressurized can be subject to control by a single pressure regulator.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

The FIGURE illustrates somewhat schematically a side view of ingot withdrawal equipment in a continuous casting machine.

DESCRIPTION OF THE DRAWING Proceeding now to the detailed description of the drawing, the FIGURE illustrates a Mold 1 for continuous casting which may be of conventional construction. A casting is withdrawn from and through the bottom of the opening'of Mold 1, and is taken up by the withdrawal and transport equipment which provides for a curved travel path of the ingot until it has been bent to continue movement in horizontal direction.

The casting is substantially liquidous as withdrawn from the mold and there is usually only a very thin so lidified shell or skin. The diameter of the liquidous core 3 of the casting reduces along the travel path, and, usually, the casting is completely solidified in the range of the last transport rolls of the withdrawal stand.

The several rolls are arranged and organized in groups. The rolls of Group A are usually joumaled and mounted for rotation ,in a fixed position, and they are not driven; they just serve as support of the withdrawal casting and providivg particular support along the first portion of the curved withdrawal path. The remaining rolls are organized in Groups B and C. This organization scheme is taken along the withdrawal path. In addition, one can regard the rolls as being divided in those which are disposed along the underside of the casting and those disposed on top thereof.

In the particular example illustrated, all of the rolls along the underside are mounted in fixed bearings so that additional pressure is not exerted by them upon the casting. The rolls on the upper side of Groups B and C are individually subjected to pressure respectively provided by hydraulic piston-cylinder units 4. The individually marked rolls are driven by DC drive motors. The motors are connected to electric current busses 6 and 7 as is schematically indicated. Electric current is supplied from the common bus to the individual motors via respective branch lines or cables 5. The common busses 6 and 7 (6 for the motor of the upper side; 7 for the motor on the underside), all run through a current measuring device which provides an electrical signal in representation of the total current drawn from the supply and fed to all of the motors.

The connection of the cylinders 4 to a source of pressure fluid is illustrated only to the extent necessary for explaining the present invention. This means that the cylinder or cylinders for urging the driven rolls along the horizontal withdrawal path against the ingot are all connected to a common feeder pipe 9 so that they receive the same pressure. The feeder pipe 9 runs through a pressure regulator 10 whose input is connected to the source of pressure supply.

In essence, the pressure regulator 10 provides some pressure reduction from a maximum pressure applied to it by the source, and the reduction is subject to automatic control. The rolls of Group C include all those driven rolls having pressure force applying cylinders which are connected to that particular pipe 9. From a different point of view, the roll Group C extends along the horizontal portion of the finally solidifying casting, and the rolls of Group B are disposed intermediate Group A and Group C and cover the last portion of the curved path of the withdrawal stand. The cylinders providing pressure forces to the upper rolls of Group B are connected to a preadjusted pressure reducer that may also connect to the same source which supplies regulator 10 with pressure fluid. Also, the cylinders of Group C providing pressure force to undriven rolls are connected individually to pressure reducers. All these pressure reducers which form part of the hydraulic means that are not connected to the common pipe 9, can be regarded as being incorporated in each of the several hydraulic devices 4. Each of them is individually adjusted so that the pressure force provided by the respective rolls balances the expansion force exerted locally by the still liquidous core portion of the casting tending to drive the skin in outer direction.

The pressure reducers for those hydraulic system cylinders that are not connected to pipe 9, are, for example, adjusted so that the contact pressure provided by the rolls is about percent of the local expansion force of the casting where engaging these rolls. One can also see that for the basic operation, the pressure reducing portion of regulator 10 provides such a pressure reduction in response to the particular constant motor current as supplied in case of normal operations to all of the roll drive motors. In particular then, a controller 1 1 responds to current flow as monitored by instrument 8 in response to normal withdrawal operation, by providing a command signal to the pressure regulator 10, so that the pressure regulator 10 provides to the line 9 basically the same pressure that is supplied to most of the cylinders pertaining to all of Group C, but imparting pressure force upon the non-driven rolls thereof.

It can therefore be seen that in the initial or normal, or regular mode of operation, there is only about 5 percent or thereabouts excess pressure exerted by the rolls upon the withdrawn'casting, so that all of the unfavorable events and possibilities of an excess pressure system do not occur. Basically, the contact pressure as provided is sufficient to maintain normal operation on the basis of the assumption that all driven rolls are in fact participating in the withdrawal operation.

Contact pressure provided by the several rolls along the withdrawal path differs only as far as absolute value is concerned. As the ferrostatic expansion force in the casting is smaller on the first roll of Group B than on the second, and still smaller then on any of the rolls thereafter and on any of the rolls of Group C needed, contact pressure increases along the withdrawal path accordingly. As the driving force is limited by the product of contact pressure force and friction coefficient, the driven rolls of Group C must provide greater traction power than those of Group B in the basic or normal mode of operation. The pulling and traction force for withdrawing the ingot from the mold results from the roller effect provided by respective two oppositely positioned rolls sandwiching the casting in between. As the solidified portions of skin of the ingot behaves analogous to a billet that is being rolled, rolling action can be minimized only by keeping contact pressure as low as possible.

The withdrawal equipment, may become partially defective in that for example, the bearings of one or severa] of the rolls are blocked, and such rolls do no longer participate in the withdrawal operation. Accordingly, the mechanical resistance offered to the still driven rolls increases, and the electric current that flows into the electric drive system increases accordingly. Therefore, the instrument 8 will proportionally indicate an increase in load current, and that increase is approximately proportional to the increase in transport resistance. As a consequence, the controller 11 responds to the increase in current consumption and provides a change in the command signal as fed to pressure regulator 10. The changed command signal causes an increase in pressure for the hydraulic system of the driven rolls of Group C, which means that the pressure reducing effect of regulator 10 is by itself reduced, to provide higher hydraulic pressure into the pipeline 9. Such operation may lead to an increase of the pressure forces provided by the driven rolls of Group C up to of the local expansion force of the ingot in that region.

The transfer characteristics of the control system as a whole requires, of course, accurate calculation and emperical determination of needed values; i.e., in any individual case it must be determined how much current increase as monitored by instrument 8 would provide a particular increase in the effective pressure in line 9. In view of the fact that instruments 7 and 11 are electrical or even electronic devices, the pressure increase in line 9 can occur practically without delay. This is an important aspect as the driven rolls must not stop, slow down or slide on the casting for any instant. Sufficient traction must be continuously provided for, as the casting continues uninterruptedly. It can thus be seen that it is important to provide a very accurate increase in the pressure force urging the drive rolls of Group C against the casting, because if the pressure force is increased too much, the damage that may occur will still occur. It should be noted that the inventive method, therefore, provides for steps that insure normal operation without excess contact pressure on the casting. Normal operation is not and must not be carried out on the basis of worst case conditions such as multiple dropouts of driven rolls. On the other hand, the pressure increase as a result of such dropout must not be excessive either. It should be noted also, that the dropout situation does not occur very frequently. It can occur but it is, in fact, an exception as was found emperically from existing multi-roll withdrawal stands. This fact makes the invention particularly useful as previously built stands used excess contact pressure just to be able to take up the rarely occurring worst case condition.

A modification of the system illustrated can readily be derived from the drawing. For example, one could use only some of the electric motors for purposes of determining current inflow, but the summation method explained above was found to be the better way of dealing particularly with the statistical distribution of possible failure. This is particularly important as individual motors themselves may, for example, become defective and require more current, which may have nothing to do with any dropout condition and nor does it relate to an increase in required traction force exerted upon the casting as a whole; in the summation individual motor defects will, in fact, not be materially noticeable.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

I claim: 1. In equipment for withdrawing a continuously cast ingot from a casting mold and along a curved withdrawal path and which includes a plurality of withdrawal rolls, some of which are being driven by motor means, and providing contact pressure upon the ingot withdrawn, there being hydraulic means for providing pressure force upon at least some of said rolls, the improvement comprising:

first means responsive to the driving power as provided by at least some of the motor means to the respective driven withdrawal rolls and providing a control signal representative thereof; and

pressure regulator means included in the hydraulic means and determining the hydraulic pressure which provides the pressure force for the contact pressure of at 'least some of said driven rolls, the pressure regulator means connected for receiving the control signal and for increasing said pressure force upon increase of the driving power needed by the motor means to cause the rolls to withdraw the ingot at a predetermined rate.

2. The equipment of claim 1 wherein the motor means includes plural electric motors and the first means is responsive to the sum of the electric current flowing thzough the motors to provide the control signals as representation thereof.

3. The equipment as in claim 1, wherein the pressure regulator means includes a pressure regulator controlling the contact pressure of driven rolls arranged along the horizontal withdrawal path for the ingot. 

1. In equipment for withdrawing a continuously cast ingot from a casting mold and along a curved withdrawal path and which includes a plurality of withdrawal rolls, some of which are being driven by motor means, and providing contact pressure upon the ingot withdrawn, there being hydraulic means for providing pressure force upon at least some of said rolls, the improvement comprising: first means responsive to the driving power as provided by at least some of the motor means to the respective driven withdrawal rolls and providing a control signal representative thereof; and pressure regulator means included in the hydraulic means and determining the hydraulic pressure which provides the pressure force for the contact pressure of at least some of said driven rolls, the pressure regulator means connected for receiving the control signal and for increasing said pressure force upon increase of the driving power needed by the motor means to cause the rolls to withdraw the ingot at a predetermined rate.
 2. The equipment of claim 1 wherein the motor means includes plural electric motors and the first means is responsive to the sum of the electric current flowing thzough the motors to provide the control signals as representation thereof.
 3. The equipment as in claim 1, wherein the pressure regulator means includes a pressure regulator controlling the contact pressure of driven rolls arranged along the horizontal withdrawal path for the ingot. 